ExtremeTech offers advice on building a back to school desktop PC, you can read it over here.
Most students are going mobile with laptops, so why would anyone want to build a desktop to bring back to school? Despite what you lose in portability, desktops have plenty to offer. A desktop will get you much more bang for your buck than an equally equipped laptop, and building your own will help you save even more. Not to mention that it's a cool project to end the summer with. And do not worry if you have never built a computer before; I'll walk you through the process..